Undertale doesn't belong to me.
Asriel stumbled underground. He felt Chara's soul desperately trying to drive him back out the barrier, back to the village of the humans. Go back! they ordered, willing his legs to move. Our job isn't done there! Go back!
No, Chara, he thought stubbornly, forcing his legs to keep going down. What would that solve? We could start a second war between humans and monsters. It isn't worth it. He felt his fresh wounds sting. He stopped and looked down at his left paw. It had two fingers blown off it, which had been painful and in fact still was. Mom's magic might not be able to heal that, he told himself, then continued.
Once Asriel stumbled into the castle that he and his parents lived in, he forcibly ejected Chara's soul. His once powerful and large form had been replaced with his usual small child body. The bright red heart floated in front of him, emitting a pleasant glow that made him feel safe. Yet he knew better. Chara had been more bloodthirsty and hostile than he'd ever known. It's time for you to rest, he silently told them, reaching his paws out. It floated in them, hovering about an inch.
"Asriel, there you are!" Toriel said, running up to him. She leaned down to wrap her arms tightly around him, but she noticed the human soul and hesitated. "Asriel, why do you have Chara's soul?"
"It's a long story," Asriel replied, then proceeded to explain everything to his mother. He neared tears when he came to the part where he and Chara had decided to poison them.
"It's alright," Toriel comforted him when he'd finished his explanation. He was barely holding back from crying. She rubbed his back. "I'm glad you told me, Asriel. Chara may be dead now, but there's nothing we can do except mourn. Stay here and hold their soul for me, will you? Don't absorb it, and don't destroy it. I'll go get a container to store it."
Asriel nodded as Toriel walked away. He sat and waited where they were right by the stairwell. It wasn't long before the goat monster returned, holding a cylinder-shaped plastic container. She popped the bottom off, took the soul from Asriel, and stuck it inside before replacing the bottom.
"Your father had gone to search for you throughout New Home, but I decided to stay here in case you came back," Toriel told her son while she did so. "He'll surely come back later when he hasn't found you, so that's when I shall tell him everything you've told me."
Asriel nodded again. "That's okay," he sniffed, having cried a small bit.
"In the meantime, would you like to help me put Chara's body into the coffin?" Toriel asked. "I had someone go order one and bring it here. I decided you deserved to be there while it happened."
"Sure," Asriel responded. "I guess so."
Toriel then lifted her paws and had pale flames appear in them. Healing flames, he thought. He dealt with the sharp sting of healing when it was touched to each and every wound he had.
"Now go wash yourself up," Toriel ordered. "Then we'll get you into some clean clothes, and get started."
Asriel nodded. He went down to the nearest stream, because his parents' castle didn't have a bathing room like some houses did, and scrubbed his snowy white coat clean of blood. His two lost fingers hadn't regrown, but fur had sprung up in their place. He even used a bit of soap to get rid of any remaining smell or stains. Then, he wrapped himself in the large towel he'd brought along and quickly padded back to the castle.
Asriel dropped his dirty outfit in the basket, then dried himself off in his room. He noticed that Toriel had left him a new, completely identical set of clothes. He changed quickly, put away the towel and soap, and found his mother in the living room - where Chara had died. His older sibling's body was surely stiff by now, and its skin had gained a greenish tone. There was a light gray coffin with a red heart on the lid and engravings underneath: "Chara." It all made him feel sick, especially since he could only wonder how they prepared a specialized coffin so quickly, but he approached anyway.
He and Toriel lifted Chara's corpse into the coffin, then set it down gently. Their head laid so calmly on the white pillow that it looked as if they were still alive, if you ignored the skin tone. Toriel lifted a paw and shut the child's amberish-brown eyes. Asriel could tell that his mother was holding back tears for the sake of looking strong in front of him.
Asriel lifted the lid and set it down over the coffin, closing it off. "What will we do with their soul?" he asked.
"Your father and I will discuss that matter later," Toriel decided. "For now, just go and play. Maybe visit Gaster, if you want. You haven't gone there in months. He's taken on an apprentice, remember? You might want to meet her."
Asriel wasn't sure he wanted to leave his grieving mother alone, but he agreed anyway and headed off to the lab in Hotland. When he returned a few hours later, he found that the coffin in the living room was gone; they must've buried it already. The container that the soul was in was also absent. He walked over to his parents' room and lifted a paw to turn the knob, but he heard arguing and decided to quietly listen in.
"For the final time, Gorey, he's fine!" Toriel said.
"No, he isn't!" Asgore shouted, and Asriel was shaken by the amount of anger that was held in it. "Those humans almost killed our son, Tori! Don't you want to do something about that?"
"No!" Toriel replied. "The humans attacked him, yes, but that's no reason to wage war."
"I don't care," Asgore insisted. "I'm killing any human that comes down here. And Chara is just an example of what happens when we get attached. I'll collect every soul of every dirty human that dares come down the mountain, and I'll free all monsters. Then, mankind will truly regret what they've done." His tone held foreboding that sent a chill down Asriel's back.
"In that case, Asgore, I can no longer be with you," Toriel told him. "I'm sorry, but you're not the monster I once knew. The true Asgore would never dream of doing what you want to do."
"Fine, go then!" Asgore yelled. "Who needs you anyway?"
They're fighting over me and my decision, Asriel realized, deciding he should step in. He turned the knob and flung the door open. "Stop!" he shouted.
Asgore and Toriel turned to look at him, their eyes wide with shock. "Asriel, go to your room," Asgore ordered gently, taking a few steps forward. "We'll have dinner in a little while, don't worry."
"I'm not worried about dinner," Asriel said. "I'm worried about you! I don't want you two to fight; you're my parents. What'll I do if you split up?"
Toriel gasped, saying quietly, "I hadn't thought of that." Then she approached Asriel as well, and knelt down so that she was at eye level with him. "Asriel, you must understand. It's for the best we part ways. Your father has changed due to what's happened, and it's dangerous to remain here."
"It isn't!" Asgore insisted. "You can go, but I'm keeping Asriel. He's the future King of the underground, after all."
"You'll just taint his soul with lies and propaganda," Toriel retorted. "I'm taking him."
"Why don't I decide?" Asriel asked, despite not wanting to. He wanted both parents, not one.
"Yes, good idea," Asgore agreed. "You can think about it for as long as you want, Asriel. Whenever you've made your decision, just tell us. Alright?"
"We won't judge you for whatever you choose," Toriel added.
Asriel nodded slowly, then his parents stood and walked over to where Chara's soul sat in its container on the desk. He sat on the floor, watching them.
"What shall we do with this?" Toriel asked, then immediately added, "You're not using it to break the barrier."
"I wasn't going to!" Asgore insisted. "But you're right, we should decide what to do."
"Maybe we can keep it with the coffin?" Toriel offered. "As a memory of Chara?"
After agreeing to it, Asgore left the room - presumably to make himself some tea. Toriel took the container and left with it. Asriel found out later that they hadn't even buried the coffin - they just put it in the empty basement, which was where the soul ended up as well.
Asriel thought long and hard about his decision. It took him an entire week, of which Toriel slept on the couch and refused contact with her ex-husband, to make his mind up. He gathered both of his parents after dinner that night and sat with them at the dining room table.
"I've decided," he sighed.
"Well? Who do you want to stay with, my child?" Toriel's magenta eyes were hopeful.
This made Asriel feel guilty, because he immediately replied with, "...Dad."
Toriel's face took on a look of disappointment and grief. "My child... if that's what you wish, then I approve. I won't try to change your mind."
"I'll come visit you!" Asriel promised.
Toriel shook her head, telling him, "I love you and will miss you dearly, but please never visit where I'm going. By leaving, I'm choosing to discard everything I don't take with me."
"Where are you going?" her son asked curiously.
"Home."